Certain radiation curable coatings and films such as those formed from the acrylates, particularly trimethylol propane triacrylate, trimethacrylate, pentaerythritol triacrylate, and hexanediol diacylate or methacrylate, are in great demand because of their rapid curing properties. However, these compounds are normally highly viscous liquids or solids and thus are unsuitable as diluents for other polymeric components of a radiation curable formulation. Indeed, such compounds themselves require the incorporation of a diluent or solvent for uniform substrate coating, control of coating thickness and curing at low temperatures. Accordingly, low viscosity monofunctional diluents are usually included in their formulations. While these diluents are reactive, they materially reduce the cross-linked density of the finished product and consequently lower abrasion resistance and ability to withstand chemical attack.
Although solvents have been used to reduce viscosity, they are detrimental in radiation curing due to their volatility which presents problems for uniform composition control unless their evaporation prior to radiant exposure is effected. Obviously, such procedure extends processing time and may pose environmental drawbacks.
To some extent, the drawbacks of high viscosity monomers can be reduced by curing at elevated temperatures. However, this alternative significantly adds to the cost of the overall operation in the expenditure of energy, temperature control and loss of more volatile components in the composition or blistering of the coating resulting from entrained volatiles.
Since acrylate monomers are not conducive to cationically induced radiation curing, they require free radical systems which are oxygen inhibited unless effected in an inert atmosphere, generally under a blanket of nitrogen. Although formulation with a photoinitiator which undergoes bimolecular reaction with a hydrogen donor minimizes the inhibitory effect of air, this benefit is realized at the expense of a greatly reduced cure rate. Also, it is found that polymerization or curing in free radical systems ceases almost immediately upon removal from the source of radiation; thus,. the cured product often contains significant amounts of unpolymerized components. Accordingly, it is an aim of research to develop a monomer having the beneficial properties of acrylates but which is amenable to radiation curing at a rapid rate by cationically induced polymerization which is not oxygen inhibited and which permits continued polymerization after removal from the source of radiation exposure.
Finally, it is noted that the unsubstituted acrylates are sensitizers and skin irritants as well as being carcinogenic, so that specialized safety precautions must be taken to protect operators from exposure. Although alkoxylation has lessened irritancy of the acrylates, their carcinogenic properties are not reduced.
The inherent deficiencies of the acrylate systems can be partially overcome by the use of epoxy resins. Epoxy resins can be polymerized by normal radiation techniques using cationic photoinitiators such as iodonium, sulfonium and ferrocene salts of hexafluorophosphate, hexafluoroantemonate and hexafluoroarsonate to produce a tack free film. Although in such formulations tack free products are almost immediately obtained, polymerization of the mixture is incomplete. It is well known that the polymerization of epoxy resins is extremely slow and requires as much as several days to achieve their ultimate physical properties. Thus, thermal curing is often employed to increase the rate of polymerization.
Certain allyl compounds also have been used as coatings; however these monomers and their oligomers are not readily curable by cationic radiation. Thermal curing is generally required to increase the rate of polymerization. While allyl ethers of polyethylene glycols are curable by UV light, they require a free radical initiated reaction which proceeds at a slow rate, generally over a period of from 2 to 10 hours in order to reach completion.
Accordingly, it is an object of this invention to overcome the above deficiencies and to provide a commercially feasible and economical process for producing radiation curable compounds which are homopolymerizable and which may be combined with other polymers or monomers not normally curable by radiation at high speed in the presence of oxygen.
Another object is to provide film forming compounds which have excellent resistance to abrasion and chemicals such as solvents, acids and basis.
Another object is to provide a simple and economical process for the copolymerization of the present compounds disposed as protective coatings on a substrate.
Still another object is to provide compounds capable of forming hydrogels by hydrolysis.
Still another object is to provide photoresist compounds capable of high image resolution upon exposure to a source of radiation.
Yet another object is to provide a substrate with an improved coating which has good adhesion and elongation properties.
These and other objects will become apparent from the following description and disclosure.